Catch FileNotFoundException when sending file body

[kachayev]

Fixes #459.

In general, we need to be more careful with the cleanup procedure and any time send-message failed for whatever uncaught reason close the connection. This specific just PR covers one of the obvious failures that the user might face. /cc #457.

[ztellman]

In general, I dislike the practice of the framework surfacing an exception to the client. If the application developer wants to do that, fine, but elsewhere we provide a bare 500 response and log the issue. I think I'd like to mirror that here.

[kachayev]

@ztellman Do you mean how error-response is implemented with stack trace printer? I just copied that from the previous implementation, but yeah... sure, that might be reworked.

[kachayev]

But let's finish with #485 first, these 2 have a lot of overlapping changes.

[ztellman]

Sorry if I misattributed where that code came from, but even if I was the one that originally did it, I think it's a mistake.

[kachayev]

@ztellman Oh, no problem with that. I just think that in such a case it makes sense to implement new behavior in a separated PR not to mess up the history of changes. WDYT?

[kachayev]

@ztellman Merged with the latest master changes (notably http-file functionality). I also made internal server error to look just the same for any exception that happened. WDYT?